Design of internal combustion engines requires delicately balancing competing requirements of low emissions and low fuel consumption. Governments generally limit the production of various emissions including NOx, smoke, soot, and unburned hydrocarbons. Reducing NOx emissions may be accomplished through various techniques. Many of these techniques require lowering combustion temperatures and in turn increasing fuel consumption.
Users of the engines in industrial environments such as work machines require that they operate over a wide range of speeds and loads while still meeting the emission requirements and while achieving reasonable fuel consumption. In particular, high speed (up to 2500 rpm), medium-bore (cylinder bores between 100 mm and 175 mm), compression ignition engines used in work machines may repeatedly cycle between a high speed, high load condition and an idle condition. Meeting emissions requirements through these transients requires a flexible combustion system. Additionally, compared with light duty operation seen in automotive engines, these engines operate a larger portion of their life in conditions that may contribute to fouling of fuel injectors.
To meet these challenges, designers must work with various tools to achieve a combustion cycle that meets the above needs. These tools include fuel injection equipment, air flow control, and design of the combustion chamber. In small bore (bore diameters of less than 100 mm) engines, air system geometries may be used to introduce air into the combustion chamber in a manner that generates swirling motion within the combustion chamber. The smaller bore engines may operate at higher speeds (in excess of 2500 rpm) and require faster mixing of fuel and air. The air system creates a swirling motion to increase mixing of fuel and air. Combustion chambers with swirl tend to have a narrower throat area compared with the overall piston diameter as shown in U.S. Pat. No. 5,000,144 issued to Schweinzer et al. on 19 Mar. 1991 and European Patent Application No. 0 911 500 published on 28 Apr. 1999. The narrow throat area creates a greater squish area between a top of the piston and cylinder head. Fuel injected into these combustion chambers is intended to enter a torroidal portion without contacting a floor portion. Schweinzer also shows a recess that allows the piston to approach a top dead center position in the cylinder without hitting a fuel injector tip. However, Schweinzer does not discuss interaction of air in the recess with performance of the injector.
Large-bore (180 mm diameter or greater), medium speed (between 900 and 1500 rpm), compression ignition engines tend to use quiescent or semi-quiescent open combustion chamber designs. These designs introduce air into the combustion chamber in a manner that generates little or no swirling motion of the gases about a central axis of the combustion chamber. Higher fuel injection pressures in these types of combustion chambers create motion to promote mixing of fuel and air. Also, finer drop sizes increase surface area exposed to air. These combustion chamber designs also have less squish area available to provide air to the torroidal section. U.S. Pat. No. 7,438,039 issued to Poola et al. on 21 Oct. 2008 discloses using an acute angle reentrant on a large bore, medium speed diesel to improve air flow in a quiescent or semiquiescent combustion chamber. Poola also teaches placing a recess near a tip of the fuel injector. The recess in Poola generally is thought of as an aid in removal of the piston from the engine for servicing. Again, Poola does not explain the interaction of air in the recess with fuel injector tip.
None of these references discuss the importance of improved air flow around the tip of the fuel injector. Without appropriate air flow, combustion characteristics of the engine may change over the its life or during certain conditions. For instance, high temperatures about the tip of the injector may cause increased fouling of the fuel injector tip over time. These changes may reduce the ability of the engine to meet both the customer requirements of low fuel consumption and the regulatory requirement of low emissions.
The current piston disclosed in this application addresses one or more aspects set out above to improve combustion in a medium-bore, high speed, compression ignition engine.